ABSCISIC-ACID INDUCED PROTECTION AGAINST PHOTOINHIBITION OF PSII CORRELATES WITH ENHANCED ACTIVITY OF THE XANTHOPHYLL CYCLE

The exogenous application of abscisic acid (ABA) to barley seedlings r esulted in partial protection of the PSII photochemistry against photo inhibition at low temperature, the effect being most pronounced at 10( -5) M ABA, This tvas accompanied by higher photochemical quenching (qP ) in ABA-treated leaves, A considerable increase (122%) in the amount of total carotenoids and xanthophylls (antheraxanthin, violaxanthin an d zeaxanthin) nas also found in the seedlings subjected to ABA, The ac tivity of the xanthophyll cycle measured by the epoxidation state of x anthophylls under high-light treatment was higher in ABA-treated plant s compared with the control. This corresponds to a higher value (0.411 ) of non-photochemical quenching (qNP) observed in ABA-treated than in control (0.306) leaves.